For many years, workers in the food industry have endeavored to increase the shelf life of a variety of food products while assuring the safety of the products for human consumption. Various sterilization techniques have been employed in the food industry to this end. Of these, the most popular has been heat sterilization particularly in conjunction with treatment of fluent products. In this regard, the maintenance of aseptic conditions guaranteeing the success of the sterilization process as well as control of other process parameters have been important to assure the effectiveness of the sterilization. For some types of food products, such as milk, careful handling of the product throughout the processing is mandatory not only for health reasons but also for the preservation of the desirable taste and other organoleptic properties of the product. These requirements have long been a significant cost factor in the marketing of the product.
In connection with denaturing the protein in milk products, that is, reduction of the Whey Protein Nitrogen Index (WPNI), high temperatures have been necessary in order to reduce the holding time and to allow greater flexibility in meeting differing product specifications such as for milk powder. It is desirable that the heat treatment be effected with minimal changes in color, taste and without production of burnt particles.
To achieve denaturing of the protein for the production of high heat milk powder (WPNI&lt;1.5), in the past, cumbersome and expensive equipment has been employed and which required that the fluid milk product be maintained at approximately 80.degree. C. for approximately 30 minutes. More recent developments have used temperatures up to 120.degree. C. for much shorter holding intervals on the order of two minutes. However, even at these higher temperatures and shorter holding times, problems persist in terms of burn-on on the process equipment surfaces resulting in shorter operating cycles and lower quality products. Also, more frequent cleaning cycles are required.
The use of high temperatures has been limited by the unavailability of suitable heaters that could correlate with the volume capacity of other downstream equipment such as evaporators and dryers which now operate at capacities of 100,000 kg/hr or more of milk feed. One manufacturer has developed a fluid distribution head for milk sterilization that allows higher flow rates and as a result higher temperatures can be used such as on the order of 143.degree. C. This translates into a shorter holding time of less than 30 seconds for the same degree of denaturation as previously attained at lower temperatures noted above. Also, at or about 143.degree. C., the bacterial killing rate is the same as for ultra high temperature and extended shelf life products and will improve the shelf life of the powdered milk product.
Ultra high heat (UHT) and extended shelf life (ESL) products require a bacterial killing rate that can be accomplished for low heat milk powders (WPNI . 6.0) by using the same high sterilizing temperatures (143.degree. C.) but it is necessary that the holding times be shorter such as on the order of 2 to 6 seconds.
Representative of the prior art are U.S. Pat. Nos. 4,310,476, Reissue 32,695, 4,591,463 and 5,544,571.
It has been found of particular importance that a fluent food product in conjunction with treatment with the sterilizing medium be handled in a manner that assures intimate contact on a molecular level between the medium such as steam and the fluent food product. As is well known, a milk product exhibits particular sensitivity to sterilization techniques. Even small temperature and other process variations during the treatment of milk can result in large changes in the taste of the product which risks rendering the product unacceptable to consumers. Moreover, marketing unsterilized milk establishes a price floor against which sterilized milk must compete. As a result, workers in this field have endeavored to provide a cost competitive technique for sterilizing milk and other fluent food products. However, while the theory of heat treatment of such products has been well tested, efficient production techniques have not been provided nor have apparatus and methods been developed that can effectively render a high quality sterilized product competitive in the market place with low temperature pasteurized liquid products.